A battery generally includes a positive electrode (cathode during discharge), a negative electrode (anode during discharge), and an electrolyte for ion transport therebetween. The electrolyte can contain one or more ionic species that act as charge carriers. Many widely available battery systems are based on cation electrode reactions, with electrodes capturing or releasing a cation from an electrolyte and balancing the charge with an electron from the external circuit. Because of its very low electrochemical oxidation/reduction potential and light weight, the element lithium (Li) is commonly used in cation based battery systems. Both lithium and Li-ion batteries are commercially available and widely used.
However, the electrochemistry of lithium metal or lithium-containing electrodes presents problems for commercial use. In one aspect, lithium metal is highly reactive and safeguards are used to store lithium in safe forms (e.g., intercalates), increasing battery weight and reducing energy density. For example, individual Li-ion batteries and Li-ion battery packs often contain expensive voltage and thermal control circuitry to shut down the battery when voltage or temperature is outside an optimal operating range.
Fluoride-anion based electrode reactions offer an alternative to lithium and lithium-ion batteries. For example, in a fluoride ion battery (FIB), an anode and cathode are physically separated from one another but in common contact with a fluoride anion conducting electrolyte. The anode and cathode are typically formed from low potential elements or compounds (e.g., metals, metal fluorides, or intercalating compositions such as graphite or other carbon based material), where the cathode material possesses a higher potential than the anode material. Fluoride anions (F−) in the fluoride anion conducting electrolyte move from the cathode to the anode during discharge and from the anode to the cathode during charge of the battery.
Notably, operation of such fluoride ion batteries requires a ready source of mobile F− in the electrolyte for operation. However, many solid-state electrolyte compositions have poor ionic conductivity at temperatures below about 200° C., resulting in significant reduction in cell performance at lower temperatures due to high cell internal resistance. Furthermore, common metal fluorides (e.g., LiF, CsF, MgF2, BaF2), transition metal fluorides (e.g., VF4, FeF3, MoF6, PdF2, AgF), main group metal fluoride (e.g., AlF3, PbF4, BiF3) and lanthanide or actinide fluorides (e.g., LaF3, YF3, UF5) are largely insoluble in organic solvents and cannot be used as liquid electrolyte components.
Accordingly, there exists an ongoing need for improved fluoride-based electrolytes for use in electrochemical applications.